Reflection-type display devices that operate in a scattering display mode with the use of a retroreflection plate have been proposed. In the scattering display mode, a change in voltage applied to a light modulation layer such as a liquid crystal layer allows the light modulation layer to switch between a transmitting state in which light is transmitted and a scattering state in which light is scattered, and the display device utilizes this to display an image or the like. A display device using this display mode does not need a polarizing plate and may therefore have an enhanced light utilization efficiency. Another advantage resides in that the viewing angle dependence is small. The structure of this type of display device is disclosed in, for example, Patent Documents 1 to 4.
The operation principle of the above-mentioned reflection-type display device is described below with reference to FIGS. 1(a) and 1(b). FIGS. 1(a) and 1(b) are diagrams illustrating display device's “black” displaying state and “white” displaying state, respectively. The “white displaying state” here refers to a display state in which the liquid crystal layer is in the scattering state. Accordingly, in the case of color display, the highest gray scale in gray scale display is called a “white displaying state” irrespective of what color is displayed. The “black displaying state,” on the other hand, refers to a display state in which the liquid crystal layer is in the transmitting state, and indicates the lowest gray scale in gray scale display.
As illustrated in FIG. 1(a), when a light modulation layer (here, scattering-type liquid crystal layer) 1 is controlled to keep the transmitting state, what a viewer 6 sees is the retroreflection plate itself. Incident light 3 from a light source 5, which is outside the display device, passes through the light modulation layer 1 and then reflected by a retroreflection plate 2 toward a direction in which the light has entered (reflected light 4b). Light from the light source 5 therefore does not enter the eyes of the viewer 6, and the “black” displaying state is obtained.
When the light modulation layer 1 is controlled to keep the scattering state, the incident light 3 from the light source 5 is scattered by the light modulation layer 1 as illustrated in FIG. 1(b). In the case where the light modulation layer 1 is a forward scattering-type liquid crystal layer, most of the incident light 3 is scattered forward by the light modulation layer 1, reflected by the retroreflection plate 2, and then exits to the side of the viewer 6 through the light modulation layer 1 in the scattering state (reflected light 4w). Scattering by the light modulation layer 1 nullifies the retroreflection of the retroreflection plate 2, thereby preventing the incident light 3 from traveling back to the incident direction. Part of the incident light 3 is scattered backward by the light modulation layer 1 and exits to the side of the viewer 6 (not shown). The display device in this case is in the “white” displaying state because part of the light that has exited to the side of the viewer 6 reaches the eyes of the viewer 6. According to this operation principle, the forward scattering as well as backward scattering of the light modulation layer 1 may be utilized effectively, and the obtained “white” display is therefore brighter.
The retroreflection plate 2 illustrated in FIG. 1 may be a layer that has retroreflection characteristics (retroreflective layer). Corner cube arrays, microsphere arrays, microlens arrays, and other arrays in which unit components (corner cubes, microspheres, or the like) are arranged two-dimensionally may be employed.
A corner cube array is an array of two-dimensionally-arranged corner cubes each of which is constituted by three faces orthogonal to one another. Light incident on a corner cube is, ideally, reflected by three faces that constitute this corner cube to return to the same direction as the incident direction. The use of a corner cube array which may have a high retroreflection rate improves the display contrast ratio of a reflection-type display device. Patent Document 3 describes that the display contrast ratio of a reflection-type display device employing a corner cube array is enhanced further by using a corner cube array that is made up of minute corner cubes as a retroreflection plate. A corner cube array made up of minute corner cubes (arrangement pitch: 5 mm or less, for example) is called herein as a micro corner cube array “(MCCA)”.
The structure of a reflection-type display device that uses an MCCA as a retroreflection plate is described next.
A reflection-type display device using an MCCA may have, for example, a structure in which the MCCA is placed across a display panel from the viewer. A structure like this where the MCCA is placed outside of the display panel (hereinafter referred to as “external MCCA structure”) is disclosed in, for example, Patent Document 4. A “display panel” herein refers to a panel structured such that a light modulation layer such as a liquid crystal layer and voltage application means for applying a voltage to the light modulation layer are formed between two opposing substrates. Of the two opposing substrates, a substrate that is on the viewer side is called a “front substrate” and a substrate on the opposite side from the viewer is called a “rear substrate”. In an external MCCA structure, the MCCA is placed on the rear side of the rear substrate.
Reflection-type display devices having a structure in which the MCCA is placed between the two substrates of the display panel (hereinafter referred to as “internal MCCA structure”) have also been proposed. For instance, the aforementioned Patent Document 3 describes a structure in which the retroreflective layer is placed between the modulation layer and the rear substrate in the display panel.
A concrete description is given below with reference to drawings on the conventional structure of a reflection-type display device having a retroreflection plate (retroreflection-type display device). The description takes as an example a reflection-type liquid crystal display device that has an external MCCA structure.
FIG. 2(a) is a plan view illustrating a state of wiring lines and electrodes on the rear substrate of the conventional retroreflection-type liquid crystal display device. FIG. 2(b) is a diagram illustrating the structure of the conventional retroreflection-type liquid crystal display device, which is a schematic cross-sectional view taken along lines II-II′ and II′-II″ in the plan view of FIG. 2(a).
A display device 100 includes a front substrate 10 and a rear substrate 12 disposed so as to be opposed to the front substrate 10. Between the substrates 10 and 12, a light modulation layer (here, scattering-type liquid crystal layer) X which is capable of taking a scattering state or a transmitting state is provided. A retroreflective layer 2 is provided on a side of the rear substrate 12 that is opposite from the light modulation layer 1.
Formed on the same side of the rear substrate 12 as the light modulation layer 1 are a plurality of thin film transistors (TFTs) 13, which function as switching elements, source lines 14, gate lines 15 for selectively driving the thin film transistors 13, and others. A plurality of pixel electrodes 16 are placed above the thin film transistors 13, the source lines 14, and the gate lines 15, with a transparent resin layer 22 in-between. These pixel electrodes 16 each define a pixel, which constitutes one unit of displaying an image. Each pixel electrode 16 is electrically connected to a drain electrode 13d of its associated thin film transistor 13 through a contact portion 24 provided in the transparent resin layer 22.
The pixel electrodes 16 are formed by using an electrically conductive material which transmits light, e.g., indium tin oxide (ITO). As illustrated in FIG. 2(b), the pixel electrodes 16 are disposed so as to be spaced apart, thus defining pixels, each of which is one unit of image displaying. On the other hand, generally, wiring lines such as the source lines 14 and the gate lines 15 are formed by using a metal material, e.g., tantalum. Though not illustrated, the wiring lines 14 and 15 are respectively connected to a source driver and a gate driver in a driving circuit which is provided on the rear substrate 12.
On the front substrate 10, a counter electrode 18 including color fitters 19, a black matrix 20, and a transparent conductive film is provided. The color filters 19 are provided for the respective pixels. The black matrix 20 is disposed between adjoining pixels and in the neighborhood of the display region so as to shield the wiring lines 14 and 15 and the thin film transistor 13 against light. Typically, the width of the black matrix 20 is set sufficiently larger than the width of each source line 14 (d>0), or substantially equal to the width of each source line 14 (d=0).
In the display device 100, by controlling the voltage which is applied between the counter electrode 18 and the pixel electrode 16, it becomes possible to switch the light modulation layer 1 between a scattering state and a transmitting state in each pixel.    [Patent Document 1] JP 05107538 A    [Patent Document 2] JP 2000-19490 A    [Patent Document 3] JP 2002-107519 A    [Patent Document 4] JP 11-15415 A